How do I make apps display stress marks for specific words on iPad?

See this

Use the onscreen keyboard to enter a diacritical mark

1. Switch to the onscreen keyboard.
2. On the onscreen keyboard, touch and hold the letter, number, or symbol on the keyboard that’s related to the character you want.
3. For example, to enter é, touch and hold the e key.
4. Slide your finger to choose a variant.
5. To hide the onscreen keyboard when you’re finished, tap .

See this for further reference

Enter characters with diacritical marks while using an Apple external keyboard with iPad – Apple Support (UK)

Adrian22KsLrFF- wrote:

how do I switch the language to stressed English? (some few words will be stressed)

There's no such "language". What exactly do you expect to happen exactly? Words to have stress marks on them without you taking any action to add them? If it is just a few words, you could try text replacements made via Settings > General > Keyboard > Text Replacement